You don't appear to be summing a time field, but rather summing a number field and then having trouble getting the result to display in time format.
sum function doesn't quite work with only one variable/field!
The sum function does work with only one field, if the field summed is from a related table such as a field in your portal.
Sum (RelatedTable::Field ) will calculate a total of the value in field for all records in related table that link to the current layout table's record.
This will produce the same result as your summary field, but you can multiple the result by 60 to get the number of seconds, select "time" as the return type and then data formatting can give you the HH:MM format that you've requested.
Alternatively, you can store your elapsed minutes in a field of type time (which records time as the number of seconds since midnight) and then your summary field can compute a total and you can use data formatting in the Inspector to format the result to display as HH:MM